Pelsser squared gaussian model

This plug-in implements the one-factor squared Gaussian model. This model assumes that the spot interest rate is a quadratic function of the underlying process, and it provides the advantage that the interest rates never become negative. The squared Gaussian is a no-arbitrage model, so it can be fitted to the initial term-structure of interest rates.The plug-in allows the user to use this model for simulation and to calibrate it to cap prices.